A quantitative evaluation of the effects of inhibitors of tubulin assembly on polymerization induced by discodermolide, epothilone B, and paclitaxel

Purpose: To determine whether inhibitors of microtubule assembly inhibit polymerization induced by discodermolide and epothilone B, as well as paclitaxel, and to quantitatively measure such effects. Methods: Inhibition was quantitated by measuring polymer formation either by turbidimetry or by centrifugation, and the amount of inhibitor required to inhibit 50% relative to an appropriate control reaction was determined. Results: The inhibitory drugs evaluated were four colchicine site agents (combretastatin A-4, podophyllotoxin, nocodazole, and N-acetylcolchinol-O-methyl ether), maytansine, which competitively inhibits the binding of Catharanthus alkaloids to tubulin, halichondrin B and phomopsin A, which noncompetitively inhibit the binding of Catharanthus alkaloids to tubulin, and the depsipeptide dolastatin 15. While relative inhibitory effects were highly variable, a few broad generalizations can be made. First, assembly reactions that were either enhanced or dependent upon all three stimulatory drugs were subject to inhibition by all inhibitors. Second, the more readily the tubulin assembled, the greater the concentration of inhibitor required to inhibit polymerization. Drug IC50 values were generally lowest with no stimulatory drug and highest when discodermolide was present; IC50 values were higher as reaction temperature increased; and IC50 values were higher as the tubulin concentration increased. Third, inhibition of assembly by inhibitors of Catharanthus alkaloid binding to tubulin changed much less as a function of changes in reaction conditions than inhibition by inhibitors of colchicine binding. Conclusions: Since there was no apparent quantitative predictability of combined drug interactions with tubulin, any combination of interest must be studied in detail.